In internal combustion engines, particularly reciprocating diesel engines, it is common to provide a governor which limits the fuel flow to the engine as a function of engine speed. The governor is designed for any particular engine to limit the speed to a rated speed which, at rated, full horsepower, will ensure that the heat generated in the engine will not be excessive in contrast to the heat dissipating capability of the cooling system for the engine. As examples, a typical rated speed may be on the order of 2200 rpm, for a rated horsepower of on the order of 250 hp. The governor also prevents an unloaded engine from running away, since its action will severely reduce the fuel flow to the engine to that small amount which is accessory for a minimal power generation to overcome drag and accessory loading of the engine at idle. A typical high idle speed may be on the order of 2500 rpm; this differs from rated speed due to the fact that the fuel/speed characteristics of the engine when unloaded are different from those when loaded.
Diesel engines utilizing accurately timed fuel injection to control the timing of combustion are of several types. One type includes a fuel rail in which fuel at the desired pressure, for proper operation of the engine under its current load and speed conditions, is made available to all of the fuel injector systems by a gear pump, the pressure output of which is very nearly a linear function of the speed thereof. In such engines, it is possible to determine the governor cutoff point by determining the speed where fuel pressure drops as a result of governor action. Such a system is disclosed in a commonly owned copending application of Goodfriend et al, Ser. No. 684,329, filed on May 7, 1976, entitled FULL THROTTLE, SPECIFIC SPEED TESTS IN INTERNAL COMBUSTION ENGINE DIAGNOSTICS.
However, in another well known type of diesel engine, the fuel is provided to the various injectors by a primary pump, which combines the function of distributing the fuel in properly timed relationship to the revolution of the engine, with the function of providing the desired quantity to each injector. The fuel is provided to a primary pump by a transfer pump, which may typically comprise a gear pump, the output pressure of which may relate to engine speed, but the output of which is not controlled or limited by the speed-responsive action of the governor. In such an engine, it is not possible to determine the governor cutoff point by measurement of fuel pressure at any point in the system. Stated alternatively, since the fuel flow to the engine is controlled by regulation of the amount of fuel passed to the primary pump, rather than by the pressure of the fuel thereat, fuel pressure does not give an indication of the action of the governor in such systems.
One suggestion has been to place a linear displacement transducer (such as an LVDT), which senses extremely small linear displacements, in a position to sense the motion of the governor. However, the mounting of any additional sensor on engines, where each engine to be tested must be provided with brackets or the like, adds to the hookup time and increases the labor cost for each engine being tested. Additionally, such sensors are too delicate for a reasonable life expectancy in a rugged shop environment.
Obviously, since the action of the governor is to limit speed, one might think of monitoring speed as an indication of governor action; however, there is no clear cut, determinable point at which speed levels off, particularly where speed measurements will vary as a function of sub-cyclic speed fluctuations of the engine itself, transducer noise and the like. Thus, a speed profile is incapable of providing an indication of governor action to any degree of accuracy which would have diagnostic meaning.